This invention is concerned with a process for the reductive alkylation of aromatic nitro-containing compounds with ketones or aldehydes. More specifically, this invention relates to an improvement in the synthesis of N-phenyl-N'-alkyl-p-phenylenediamines which are useful as rubber antioxidants and antiozonants. At present N-phenyl-N'-alkyl phenylenediamines are produced from p-nitrodiphenylamines which are reduced and then alkylated. Other processes for the production of p-nitrodiphenylamines are described in U.K. Pat. Nos. 798,148 and 834,510; German Pat. No. 185,663; U.S. Pat. Nos. 3,155,727 and 4,155,936. P-nitrodiphenylamine has the generic formula: ##STR1## wherein R and R.sub.1 are selected from the group consisting of hydrogen radicals and alkyl radicals of 1 to 9 carbon atoms; R.sub.2 and R.sub.3 are selected from the group consisting of hydrogen radicals, alkyl radicals from 1 to 9 carbon atoms, alkoxy radicals of 1 to 9 carbon atoms and cycloalkyl radicals of 5 to 6 carbon atoms.
Presently, p-nitrodiphenylamines are synthesized by reacting (1) para-halonitrobenzenes conforming to the following structural formula: ##STR2## wherein X is a halogen selected from the group consisting of chlorine and bromine; and wherein R and R.sub.1 are defined above; (2) with a primary aromatic amine of the following structural formula: ##STR3## wherein R.sub.2 and R.sub.3 are defined as above; (3) in the presence of a neutralizing agent, selected from the group consisting of alkali metal salts, oxides of alkali metal salts and alkali metal hydroxides; (4) in the presence of a catalyst in a concentration of at least 0.1 parts by weight per hundred parts per weight of the para-halonitrobenzene; (5) at a temperature of 170.degree.-250.degree. C.; (6) at a pressure of from atmospheric to about 3000 kPa (kilopascals) and (7) with an excess of primary aromatic amines of from 3 to 300 percent.
Stage I of the reaction is the formation of the 4-nitrodiphenylamine. Stage II involves the reduction to 4-aminodiphenylamine while Stage III is the reductive alkylation to provide the final product.
The current Stage II commercial reaction involves the reduction of 4-nitrodiphenylamine to 4-aminodiphenylamine with a copper chromite catalyst at about 200.degree. C. ##STR4##
Stage III is the alkylation of 4-aminodiphenylamine with MIBK (methylisobutyl ketone) at 185.degree. C. using a nickel catalyst which produces N-phenyl-N'-alkyl-p-phenylenediamine. ##STR5##
These presently accepted means of synthesis are usually conducted at temperatures lower than 205.degree. C. and at times in excess of 12 hours. Further, the presently accepted commercial synthesis route suffers from poor product quality in that a fair amount of tars and by-products are present in the final product and the requirement of three reaction stages. The improvement of the present application is characterized in that a polymer bound anthranilic acid-palladium complex plus a bound sulfonic acid resin are used as catalysts in a combined Stage II and III reaction.
To one skilled in the art it would be apparent that the catalyst system of the present invention would be useful for the preparation of numerous compounds.
U.S. Pat. No. 4,155,936 is herein incorporated by reference and made a part hereof. Specifically, U.S. Pat. No. 4,155,936 is concerned with the incorporation of solubilizing agents in the reaction mixture to reduce reaction times and improve yields of Stage I.
The present invention provides a solution to the problems of long reaction times, limited number of suitable catalysts, environmentally unsound effluents from the reaction and multiple reaction stages.
The patents and literature cited do not suggest or disclose that unexpected improvements and simplification in the synthesis of N-phenyl-N'-alkyl-p-diphenylamines can be obtained. More specifically, the process of the present invention provides a means for avoiding metal ions in the waste water effluent, improved efficiency of the reaction, and simplification of the procedure.